1. Field of the Invention
This invention relates to an optical data storage accessing apparatus which employs a head for reading or writing data on an optical data storage medium, and more particularly, to an improved optical data storage accessing apparatus which employs two optical systems, one for data recording and the other for data reading.
2. Discussion of the Related Art
An optical card accessing apparatus which employs a laser diode for data recording and a light emitting diode (LED) for data reading is well known. Data recording light (hereinafter referred to as recording light) which is linearly polarized and emitted from the laser diode of a recording optical system is collimated by a collimating lens. Data reading light (hereinafter referred to as reading light) emitted from the LED of a reading optical system is collimated by another collimating lens. The recording and reading light are projected through a polarization beam splitter. The polarization direction of the recording light is set such that all of the recording light propagates through the polarization beam splitter. About fifty percent of the reading light emitted from the LED joins the recording light in the polarization beam splitter and reflects off the splitter. The joined recording and reading light propagate through a non-polarization beam splitter and further through a condenser lens to reach the optical card. The nonpolarization beam splitter is used to separate the reading light reflected by the optical card from the recording light.
The recording light forms a first light spot SP.sub.1 for data recording on a data storage area of the optical card. The reading light forms a second light spot SP.sub.2 for data reading, which follows the first optical spot on the data storage area.
In this apparatus, the optical card is moved in a given direction relative to the optical head and data (a data unit is called a pit) are thermally irreversibly recorded on the data storage area using the first light spot SP.sub.1. After the data recording operation, the second light spot SP.sub.2 is projected onto the data storage area. A light receiving system, which includes a condenser lens and a light detector, receives the reading light reflected by the data storage area to check for pits or to detect pit lengths. Thus, the data recording and monitoring of the data can be done during a one-directional movement of the card.
The data are sometimes incorrectly recorded because a laser diode for the recording operation often deteriorates with time or due to changes of temperature, and the data storage area of the card often collects dust. An apparatus with two optical systems can quickly check for incorrect data because the apparatus monitors the recorded data immediately after the recording.
The first light spot SP.sub.1 for data recording is required to have at least a predetermined energy density to form thermally irreversible pits on the data storage area of the optical card. In the conventional systems, however, the energy density of the first light spot SP.sub.1 often becomes lower than the predetermined density because only about fifty percent of the recording light can pass through the nonpolarization beam splitter. Consequently, the data are often incorrectly recorded. If a second polarization beam splitter is used instead of a nonpolarization beam splitter, the energy density of the recording light is increased. In this case, however, since about fifty percent of the reading light is reflected by the first polarization beam splitter and almost all of the remaining reading light is further reflected by the second splitter, almost all of the reading light is hindered from reaching the optical card. Therefore, no data can be read.
Thus, use of a nonpolarization beam splitter for separating the reflected reading light lowers the recording efficiency. Moreover, a laser diode with higher power is required to offset the lowered recording efficiency. However, since the price of a laser diode exponentially increases as the rated power thereof increases, the cost of the apparatus dramatically increases even with only a slight increase in the power of the laser diode.